1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device. More specifically, the invention relates to a one-time programmable (OTP) memory using irreversible storage elements to which information can be written only once.
2. Description of the Related Art
Recently, an OTP memory using storage elements of an insulating film breakdown type, such as an electrical-fuse element (e-fuse element) and an anti-fuse element, has been proposed as an irreversible storage element. The OTP memory stores information by applying a high voltage that exceeds the maximum rating to a semiconductor element having a metal oxide semiconductor (MOS) structure as an irreversible storage element and electrically breaking a gate insulating film of the semiconductor element. The OTP memory using the e-fuse element as a storage element is used to store information “0” in the e-fuse element whose insulating film has not been broken and information “1” in the e-fuse element whose insulating film has been broken. The OTP memory is also used to store information for replacing a defective element with a dynamic random access memory (DRAM) or the like.
Conventionally, an OTP memory using an optical-fuse element whose storage node is broken by laser beams as a storage element has been often used. The e-fuse element has originally been used as an alternative to the optical-fuse element. The following example has been proposed: one storage cell is configured for each e-fuse element to include a sense circuit for reading information and a latch circuit for latching the information. See, for example, H. Ito et al., “Pure CMOS One-Time Programmable Memory using Gate-Ox Anti-Fuse,” Proceedings of the IEEE 2004 Custom Integrated Circuits Conference, pp. 469-472.
The OTP memory using the e-fuse element as a storage element is a nonvolatile semiconductor memory device to which information cannot be rewritten. It has the problem that it cannot easily be tested like the OTP memory using the optical-fuse element as a storage element. The e-fuse element is a storage element to which information can be written only once by varying electrical properties irreversibly. The OTP memory therefore has the following problem. Neither a test on whether information can correctly be read out of the e-fuse element nor a test on whether information can correctly be written to the e-fuse element can be conducted by actually writing information to the e-fuse element.
The OTP memory has also the following problem. The amount of information of the e-fuse element (a difference in resistance between the e-fuse element whose gate insulating film has been broken and the e-fuse element whose gate insulating film has not been broken) is easy to reduce because a high voltage in write mode is applied to the nonselected (unprogrammed).
As described above, the OTP memory using an irreversible storage element, especially the OTP memory using the e-fuse element as a storage element has pursued an effective method of preventing the amount of information from decreasing and conducting a test on whether information can be read from/written to a storage element without writing information to the storage element actually.